A partial implant for the ulnar head has been described in an article of K. D. Gordon et al. with the title “Kinematics of ulnar head arthroplasty” that was published in The Journal of Hand Surgery, Vol. 28B, No. 6, December 2003. It comprises a hemispherical head intended as a replacement for the articular segment of the ulnar head, that is, the lateral segment of the distal end of the ulna articulated with the radial head. The hemispherical head is fitted onto the end of a stem, and rigidly fastened there with a screw, the stem in turn being intended to be rigidly fastened inside of a hole formed in the ulna so as to hold the implant in place.
Contrary to a total ulnar head implant, such a partial implant allows the triangular ligament linking the ulnar head and radius as well as the ulnocarpal ligament linking the ulna and the wrist, to be preserved and the interostal membrane linking ulna and radius, to be kept under tension. Hence, the stability of such a partial implant is distinctly superior to that of a total implant.
The ulnar head is known to be subject, more to transverse than to axial loads. It has actually been observed that only about 15% of the axial loads that are applied to the distal end of the fore-arm are taken up by the ulna, the other about 85% are taken up by the radius. The transverse loads are particularly high during fore-arm rotations (pronation, supination). For illustration, FIGS. 6 and 7 are representations of the axial loads taken up by ulna and radius in a vertical position of the hand (FIG. 6), and of the transverse loads mutually applied by the ulnar head and radial head in a horizontal position of the fore-arm (FIG. 7).
In the case of the partial implant mentioned above, the transverse loads taken up by the head of the implant are transmitted to the stem. It follows that the remaining segment of the ulnar head as well as the surrounding ligaments are no longer subject to the loads taken up by them initially. Not any longer under pressure, the remaining segment of the ulnar head is condemned to undergo progressive decalcification, and the surrounding ligaments, to lose their vigor.